The present invention concerns a watch with a mechanical movement whose escapement is driven via a constant force device.
In mechanical horology, the expression “constant force device” has been used for more than a century to designate a member inserted in the gear train connecting the main spring to the escapement and comprising a permanently loaded intermediate spring, in order to transmit a relatively constant torque to the escapement, corresponding to the tension of the intermediate spring. This tension oscillates a little, but within a small margin and its mean value remains constant over time. Thus, variations in the state of winding of the main spring do not make the forces applied to the mechanical oscillate vary, thus the amplitude and frequency of the mechanical oscillator are more stable.
The constant force device temporarily locks the part of the gear train located upstream, and then periodically releases it, which tightens the intermediate spring. In generally, the release rate is once per second and this allows a seconds indicator hand, commonly called the independent seconds, to jump forward at this rate. Constant force devices are thus almost always combined with the seconds wheel, with an independent seconds display. Various examples of such combinations and the operation thereof are disclosed in Patent publication nos. CH 47 297, CH 98 828, CH 120 028, EP 1 319 997 and EP 1 528 443. The last cited publication further provides that the regulator member driven by the constant force device could be a tourbillon, which is secured to the output wheel of the constant force device.
The stop and periodic release mechanism of the independent seconds device must be controlled from the wheel that follows the seconds wheel, thus the escapement wheel. The first member to be released is generally called the “flirt”, because at the moment that it is released, it rotates very quickly to complete one revolution or a fraction of a revolution, until it is locked again. The element that locks the flirt is generally a star wheel with N branches, completing one revolution in N seconds in order to release the flirt once per second. Typically, as shown by CH Patent No 47 297, this star wheel is secured to the escapement wheel, since the latter rotates at a speed (one revolution in six seconds) which corresponds to an acceptable number of branches of the star wheel. The instantaneous rotation and consecutive abrupt stop are accompanied by the entire gear train located upstream and by the display members connected thereto. The inertia of these elements means that the stop mechanism transmits non-negligible shocks to the escapement via the star wheel. A substantial part of the advantages due to a constant force driving the escapement is therefore lost.